Friday, February 1st, 1861.

CAPTAIN E. G. FISHBOURNE, R.N. C.B. in the Chair.

THE BRITISH TIDES.

By THE REV. H. M. GROVer.

WHEN these papers were written, Baron Humboldt's "Cosmos" was still in the hands of the public, and led the popular mind upon most of the physical obscurities of the day. One of the doctrines promulgated there was, that the great tide-wave of the earth circulated from east to west with the moon's diurnal course, instead of from west to east, in concurrence with her real motion in her orbit.* This doctrine appeared to be wrong, and the recent Admiralty chart is wholly confirmatory of the opposite hypothesis, being framed to show a tidal movement of the Atlantic from the westward and southward. The Atlantic tide moves across that ocean in a direction nearly from south-west to north-east, and that flow being repressed at an equal angle from the western coast of Ireland, the momentum of that vast movement will be directed from the coast towards the north-west, and acting by a repressive force upon the advancing wave, will make the course of that wave take a more direct northern course in those latitudes which lie beyond Ireland and the northern isles of Scotland. No doubt that repressed momentum will give way by degrees to the main drift of the Atlantic tide as it proceeds northward; but the effect of the new direction may account for the slow advances of the tide along the western coast of Ireland, from Bantry Bay to Achil Head, from whence there is found an entire contemporaneity of the tides with the very northernmost point of the Scotch islands. The tide, which takes 2h. 20m. to traverse the Irish coast from its southern point to the Head of Achil, 150 miles distant, along which it observes a pretty equal rate of progression, should occupy 5 hours in reaching the Isle of Lewis at the north of Scotland, which is 400 miles from the River Shannon, if there were no diversion of the Atlantic wave in its approach to that more northern district; but delayed beyond its normal interval upon the Irish coast, it accelerates its culmination beyond that point, wherefore, by the cessation of the repressive action, a comparatively still water is generated. We regard it in this point of view, that taking the great Atlantic wave to continue under that state of repression which it meets with from the Irish coast, for a few degrees beyond the northernmost point of Scotland, the sudden adjustment of a common high water along the Scotch margin may be taken to proceed from that interception of the regular tide-wave in that latitude. For the course the repressed tide-wave would take is shown in the annexed diagram; and

* Cosmos, vol. i. p. 229, Sabine's translation,

though only such as the result of observation affords, it admits of the certain inference that, from some cause or other, the normal direction of that wave has been subjected to a diversion from its proper flow from the south-west (as found in the south of Ireland). The state of the Atlantic tides is well understood. But the result of calculations made in connexion with the present investigation, was to show that two contemporaneous high-water waves cross the Atlantic, at an interval of about 1000 miles. One of these waves is found to extend from Senegal in a line towards the American coast about North Carolina or Virginia, passing a few miles in advance of the Bermuda Islands to the northward, while the other or anterior wave will be found from a point about 100 miles south of Madeira, in a line towards Newfoundland; or a few leages to the northward of Halifax in Nova Scotia. The interval between these two waves is taken to denote the length of the Atlantic tide-wave in its ocean bed; and that interval in its mean dimension appears to be about 1000 miles. We say the mean distance, because the spaces between the same two waves differ on the opposite sides of the Atlantic; that on the eastern side being greater by about 200 miles than is found on the western. This probably arises from the sluggish state of the ocean in those eastern longitudes, while the flow of the Gulf Stream in the west gives animation to the ocean, accelerates the flowing waves, and shortens the intervals between them. Upon this point, however, the mere suggestion is offered, to which inquiry may be fairly directed.

The state of the tides on the Irish coast and in the neighbourhood of the Azores is wholly confirmatory of the interval here assigned to these ocean waves; for the tides at Bantry Bay, and at Fayal in the Azores, are very nearly contemporaneous tides: the distance between those two points being fourteen degrees of latitude; and again showing, therefore, the same proximate interval between the two contemporaneous waves in the ocean of about 1000 miles. The next point we have to discuss is the approach of this Atlantic wave towards the North Sea and the eastern waters of Great Britain. This appears to be from the north-west, and from that part of the ocean which is beyond the Shetland Islands. What we imagine to be the true state of the case is, as has been already suggested, that the Atlantic wave proceeds many leagues beyond Scotland before it recovers from its repression from the Irish coast, and in some new latitude. That repression being exhausted, a lateral and backward swell takes place from that bank of repressed waters, towards Norway and the Orkney and Shetland Islands. This is inferrible, because-while the traversing of the tide occupies only 21 hours from the south of Ireland to Lewis Island, at the north of Scotland, 400 miles-there is an interval of full 4 hours between the tide of Lewis Island and the Orkneys, only 200 miles distant to the eastward; and there the tide flows through those islands with a most terrific violence, such as a rush of waters by the break up of a swollen bank would occasion, in addition to the tidal movement. As the tide appears upon the Norway coast also at an earlier time at Drontheim than at Bergen, it is by that shown that this arm of the ocean tide comes towards those regions in a direction from the north and west, and not, as in Ireland, from the southwest.

Our time will not permit any analysis of these periodical arrivals; but

the lines in the diagram No. 1, will indicate the proximate conditions under which these waters have their tidal movements. The assumed bank of elevated waters, and its break-up into an ebb or refluent flow from its eastern limb, will be better understood when we have considered, as we purpose to do, the effects of the repelled momentum of these Atlantic tides upon the coast of Brittany by reflection from the Cornish coast, and the consequent flow of a new tide into the Brighton Channel from the broken bank of elevated waters in the bay of St. Malo and its adjacent districts.

The tide-wave of the North Sea seems then to be generated by a southeastward reflux out of the Atlantic wave from a position some few leagues to the north of Scotland, and to enter the North Sea with a reduced momentum; as we may conclude from the length of its tide-wave, which we find to be about 670 miles, in lieu of the 1000 miles of the Atlantic wave. This moderated tide-wave is found then at the opening of the North Sea, marshalled from its refluent sources in one long array from the Scotch to the Norwegian coast. The Aberdeen and Bergen tides are contemporaneous tides, and a continuous line of high water lies between these two places at the same hour, and forms the incipient tide-wave of the North Sea. The tables show the tides at Aberdeen to be 56 minutes before the London standard, and those of Bergen, on the coast of Norway, 37 minutes after the standard; which, by a due allowance for the difference of longitude, shows the two tides to be within four minutes of contemporaneous occurrence. From this starting point the course of the North Sea tide-wave may be traced step by step along the whole length of that inclosed sea. For, taking the Aberdeen high water to happen at 56 minutes before 12 o'clock at night, the same wave will reach Shields about Oh 15m a.m.: it will reach the Humber at 2h 25m a.m., and Bremen, exactly opposite, on the eastern side of the North Sea, about 30 minutes later by the Bremen clocks: but, allowing half-an-hour for the precurrence of Bremen time to that of the Humber, it being about 8 degrees to the east of that longitude, those tides also will appear to be contemporaneous tides, as are the Aberdeen and Bergen. See Diagram 1.

Continuing southward on the English coast, the same tide-wave will be found in full culmination at King's Lynn at 3h 40m a.m.; but at this point it stays its progress to the south on that coast, for it does not appear at Lowestoft (only 30 miles further to the south than Lynn) till 41 hours after the tide at that place. In lieu of a direct onward course on the English side of the sea from this point, the course of progress is continued upon the coast of Holland, where the same wave will be found at the Hague, about an hour after the Lynn tide, or about 4h 40m a.m.; and, from that wave, by a ripple or reverted course, it reaches Lowestoft and the Norfolk coast about 8h 20m a.m., or about 4 hours after the Lynn tide. It seems quite obvious, from the course of succession here shown in the tides of these places, that the Norfolk promontory forms a direct breakwater to the western limb of this tide-wave, of which it turns the whole volume, under a single impulse, towards the east and to the coast of Holland. This deviation does not appear as a modification only of the course of part of its waters-such as a stream suffers in passing a buttress, which it avoids by a current round the obstructing point; but it appears rather as a diversion of the whole momentum of the advancing wave into VOL. V.

D